The present invention relates to a control apparatus for an AC generator, and more particularly, to a control apparatus for a vehicular AC generator which cuts off a field current when the generator stops its operation or when a signal in the form of a current from the generator is stopped.
A typical example of an AC generator having a known control apparatus is illustrated in FIG. 2. In this figure, an AC generator in the form of a vehicular AC generator, generally designated by reference numeral 1, includes an armature coil 101 in the form of a three phase armature coil comprising three coil elements arranged in a star-like manner and connected to each other at one end, and a field coil 102.
A rectifier in the form of a full-wave rectifier, generally designated by reference numeral 2, is connected to the armature coil for rectifying the output thereof. The full-wave rectifier 2 has an output terminal 201, a ground terminal 202 connected to ground, and three input terminals 203 each connected to the other end of a corresponding coil element of the armature coil 1. The full-wave rectifier 2 also includes three pairs of rectifying diodes, each pair comprising two diodes serially connected to each other at a junction or input terminal 203. The three pairs of serially connected diodes are connected in a parallel relation with each other at their opposite ends to the common output terminal 201 and the ground terminal 202, respectively.
A voltage regulator, generally designated by reference numeral 3, is connected to the field coil 102 of the AC generator 1 and to the output terminal 201 of the rectifier 2. The voltage regulator 3 includes a pair of voltage dividing resistors 301, 302 serially connected to each other between the output terminal 201 of the rectifier 2 and ground, a Zener diode 303 having a cathode connected to a junction between the resistors 301, 302, a control transistor 304 having a base connected to an anode of the Zener diode 303 and an emitter connected to ground, a base-current supplying resistor 305 having one end thereof connected to a collector of the transistor 304, a switch 306 in the form of a power transistor having a base connected to a junction between the collector of the control transistor 304 and the base-current supplying resistor 305, an emitter connected to ground and a collector connected to the output terminal 201 of the rectifier 2 through the field coil 102 of the AC generator 1, and a suppression diode 307 having a cathode connected to the collector of the power transistor 306 and an anode connected to the output terminal 201 of the rectifier 2.
A storage battery 4 is connected between the output terminal 201 of the rectifier 2 and ground. The storage battery 4 is also connected at a positive terminal or electrode thereof through a key switch 5 to the other end of the base-current supplying resistor 305 of the voltage regulator 3 and directly to one end of the voltage dividing resistor 301.
A starter switch 6 and a starter 7 in the form of a motor, which are series connected to each other, are connected between the positive and negative electrodes of the storage battery 4.
In this connection, the rectifier 2, the voltage regulator 3, the storage battery 4, the key switch 5, the starter switch 6 and the starter 7 constitute a known control apparatus.
In operation, when the key switch 5 is first turned on, the storage battery 4 does not have sufficiently high voltage to make the Zener diode 303 conductive, so the control transistor 304 is non-conductive or turned off. Thus, at this time, a current begins to flow from the storage battery 4 to the base of the power transistor 306 through the base-current supplying resistor 305, making the transistor 306 conductive. As a result, a current is supplied from the storage battery 4 to the field coil 102 of the AC generator 1 by way of the now conductive power transistor 306. Subsequently, when the starter switch 6 is turned on, the starter 7 is energized by the storage battery 4 to start rotation, whereby an unillustrated engine operatively connected to the starter 7 is started. With the start of the engine operation, the AC generator 1 is driven to rotate by the engine so that it begins to generate electrical power. However, as long as the output power of the AC generator 1 is less than a predetermined level, both the Zener diode 303 and the control transistor 304 remain non-conductive, so the power transistor 306 continues to be held conductive. Accordingly, the field current supplied from the battery 4 to the field coil 102 increases, raising the output voltage of the AC generator 1. When the generator output voltage exceeds a predetermined level, the Zener diode 303 becomes conductive so that a current is supplied from the battery 4 to the base of the control transistor 304 through the now conductive Zener diode 303. As a result, the control transistor 304 is turned on and hence the current supply from the battery 4 to the base of the power transistor 306 is stopped, thus turning it off. Consequently, the current supply to the field coil 102 decreases to reduce the output power of the generator 1. With a repetition of the above operations, the output voltage of the generator 1 is regulated to a prescribed value under the action of the voltage regulator 3.
With the known control apparatus as constructed above, however, in the event that the engine is stopped for an extended period of time with the key switch 5 left on, or in the case that the engine stops for some reason during operation of the vehicle and the vehicle is left in that state, the storage battery 4 continues to supply a field current to the field coil 102 while there is no output voltage produced by the generator 1. Thus, due to the continued discharge, the battery 4 at last discharges to result in a so-called dead battery. Moreover, with a control apparatus of the type in which heat generation of the voltage regulator 3 is cooled by streams of air which are generated by a fan driven by the generator 1, there will be no cooling air provided by the fan when the generator 1 is stopped. As a result, the temperature of the voltage regulator 3 rises abnormally, causing thermal damage thereto.